Acetylcholine receptors in a cell membrane control the membrane potential of the cell according to the concentration of acetylcholine exposed to them. Depolarization of the cell is caused by changes in the membrane permeability to sodium and potassium caused by conformational changes in the receptor resulting from the binding of acetylcholine. Acetylcholine receptor is present at neuromuscular and some neuronal synapses, and its response determines whether a muscle fiber contracts or whether a neuronal signal is propagated. It is the site of action of several lethal toxins and its function is involved in several diseases (e.g., myasthenia gravis). It serves as a general model for less available neurotransmitter receptors. There is some evidence for multiple ligand binding sites and multiple conformational changes in these responses. Various possible conformation changes mediated by a particular binding site characterized by a neighboring reducible disulfide group are the center of these studies. Membrane vesicle preparations containing the acetylcholine receptor prepared from electric fish particularly Electrophorus electricus will be used. Rapid mixing, flow techniques, quench flow and stopped flow, will be used to measure the receptor mediated transmembrane ion flux and its ligand induced desensitization and also the rate of alkylation of the receptor with an affinity ligand. Correlation of these results will give information on the types of structural changes taking place. The relationship between the processes of channel opening and desensitization will be investigated by similar measurements made with various modifications in the receptor protein structure and the specific ligands used.